In recent years, a great progress has been made for an organic thin-film transistor (hereinafter referred to as “organic TFT”). Organic TFTs are advantageously used in that the organic TFTs can be produced at a low temperature as compared with inorganic TFTs, and an inexpensive resin substrate as a flexible substrate can be used. Because of these advantages, organic TFTs are expected to be applied to a low-cost IC technology for a smart card, electronic tag, display, or the like.
A general organic TFT is composed of a substrate, a gate electrode, a gate insulating film, a source electrode, a drain electrode and an organic semiconductor. By changing a voltage applied to the gate electrode (gate voltage, Vg), the quantity of electric charges on the interface between the gate insulating film and the organic semiconductor is made excessive or deficient, and the value of a drain current (Id) flowing among the source electrode, organic semiconductor and drain electrode is changed, whereby the TFT is switched on and off.
As physical values for indicating performance of an organic TFT, a mobility, ON/OFF ratio and gate threshold voltage are used. The mobility is generally calculated from the gradient of the Id1/2-Vg curve in a saturation region in which Id1/2 and Vg are in a linear relation, and indicates the degree of easiness in allowing a current to flow. The ON/OFF ratio is represented by the intensity ratio of the minimum Id to the maximum Id when changing the Vg. The gate threshold voltage is defined by the X-intercept of a straight line in contact with the Id1/2-Vg curve in the saturation region, and indicates the gate voltage at which the TFT is switched on and off.
As target values of characteristics of an organic TFT, values of an amorphous Si TFT used for an existing active matrix liquid crystal display apparatus are assumed. Specifically, the mobility is 0.3 to 1 cm2/Vs, the ON/OFF ratio is 106 or more, and the gate threshold voltage is 1 to 2 V.
Recent researches have revealed that characteristics of an organic TFT are associated with crystallinity of an organic semiconductor. For example, A. R. Brown, D. M. de Leeuw, E. E. Havinga, and A. Pomp, “Synthetic Metals”, Vol. 68, pp. 65-70, 1994 discloses that an organic TFT using an amorphous organic semiconductor cannot have a high mobility and a high ON/OFF ratio in combination.
In order to improve characteristics of an organic TFT, various attempts of improving the crystal state and orientation of an organic semiconductor have been made. One example is an attempt of improving crystallinity of an organic semiconductor by placing an underlayer under an organic semiconductor layer. Japanese Patent Application Laid-open No. H07-206599 discloses a method of orienting an organic semiconductor of an oligothiophene compound or the like using a polytetrafluoroethylene (PTFE) oriented film as an underlayer. However, since solid PTFE is slided at a constant pressure to form the PTFE film, it is difficult to produce a substrate with a large area.
Y-Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, “IEEE Electron Devices Letters”, Vol. 18, No. 12, pp. 606-608, 1997 discloses a method of obtaining a high-performance organic TFT by coating the surface of a gate insulating film with octadecyltrichlorosilane as a vertically oriented film and then forming two pentacene-deposited film layers. However, this method can be applied to only a surface having a basic functional group such as silicon oxide, and is therefore less versatile.
Japanese Patent Application Laid-open No. 2001-9410.7 discloses an organic semiconductor device produced by a method in which a fluorine polymer layer with a film thickness of 0.3 to 10 nm is formed on the surface of a gate insulating film by dipping, and a crystalline organic semiconductor is formed thereon. However, it is considered that, in this method, crystals of the organic semiconductor are not sufficiently oriented because the crystals have two peaks in the wide-angle X-ray spectrum, and the device does not have satisfactory characteristics.
U.S. Pat. No. 6,433,359 discloses a method of improving the mobility of an organic TFT by treating the surface of an alumina gate insulating film with alkyl phosphate. Since there are limitations to the gate insulating film to which this method can be applied, the method is also less versatile.
M. Yoshida, S. Uemura, T. Kodzasa, T. Kamata, M. Matsuzawa, and T. Kawai, “Synthetic Metals”, Vol. 137, pp. 967-968, 2003 discloses a method of improving the growth of crystals of an organic semiconductor by forming a polymer layer of polymethyl methacrylate or the like on an inorganic gate insulating film. However, an organic TFT obtained by this method has characteristics that are inferior and insatisfactory for practical use.
International Publication Number WO 03/041185 A2 discloses an organic TFT comprising a substantially nonfluorinated polymeric layer having a thickness less than about 400 angstrom interposed between a gate dielectric and an organic semiconductor layer.